The present invention relates to an improved apparatus and process for the preparation of sodium monofluoro-phosphates, utilizing a melting crucible of a platinum/rhodium alloy.
Alkali metal monofluorophosphates are prepared by melting a mixture of an anhydrous alkali metal fluoride and an alkali metal meta-, pyro- or polyphosphate at temperatures of about 800.degree.-1000.degree. C. in melting vessels of platinum, silver, iron or graphite (German Patent 1,792,648). Because of the high heat stability, hardness and chemical resistance, crucibles of a platinum/rhodium alloy are preferably used for this purpose.
In spite of the high resistance of the crucible materials used, however, holes and cracks are found in the crucible wall after a certain operating period, and these make it necessary to replace the crucibles. The use of less expensive crucible material, for example iron or graphite, has not proved appropriate, since these show increased wear, whereby the end product is contaminated.